1. Field of the Invention
The invention relates to a vehicle and to a control method of a vehicle. More specifically, the invention relates to responsiveness diagnosis of an air-fuel ratio sensor that is installed in a vehicle.
2. Description of Related Art
Internal combustion engines are controlled on the basis of a detection value by an air-fuel ratio sensor that is disposed in an exhaust passage of the internal combustion engine. For example, there is a conventional control method in which a fuel injection amount is compensated on the basis of a detection value by an air-fuel ratio sensor, in such a manner that the air-fuel ratio of the internal combustion engine comes close to a stoichiometric air-fuel ratio.
Japanese Patent Application Publication No. 2008-95627 (JP-2008-95627 A) discloses an air-fuel ratio control device for internal combustion engines that has a function of compensating variability of an output characteristic derived from manufacturing tolerances, changes over time and so forth in an air-fuel ratio sensor. In JP-2008-95627 A, the supplied air-fuel ratio is deliberately altered periodically to richer or leaner ratios while the vehicle is traveling. An output characteristic compensation value of the air-fuel ratio sensor is calculated on the basis of a comparison between a change amount of the output of the air-fuel ratio sensor, and an oscillation width of the air-fuel ratio, arising from periodic changes in the supplied air-fuel ratio.
In JP-2008-95627 A, the air-fuel ratio must be modified according to the intended oscillation swing during operation of the internal combustion engine (during fuel combustion), with a view to detecting accurately the output characteristic of the air-fuel ratio sensor. However, the actual air-fuel ratio may fail to be modified as intended, due to the influence of combustion variability, and hence the output behavior of the air-fuel ratio sensor may in some instances fail to be accurately diagnosed. Moreover, the air-fuel ratio is modified during actual driving of the internal combustion engine. This may entail worse emissions as well as a poorer drivability on account of output fluctuations in the internal combustion engine.